Update leaflet-providers-0.0.2.js

Add Esri's Wold Canvas - Light Gray Base
http://server.arcgisonline.com/ArcGIS/rest/services/Canvas/World_Light_Gray_Base/MapServer
Update demo.html

Add Esri's gray canvas to basemap demo
Update leaflet-providers-0.0.2.js

Add more Esri basemap options
Update leaflet-providers-0.0.2.js

Add WorldStreetMap options for full attribution
Update demo.html

Add new basemap options to demo
Update leaflet-providers-0.0.2.js

Remove option for NatGeoTopo
Update demo.html

Remove option for NatGeoTopo in demo
Merge branch 'master' of https://github.com/GeoSteve/leaflet-providers into GeoSteve-master